1. Field of the Invention
The present invention relates to an arrangement for transmitting speech, and more particularly to an arrangement for transmitting speech via a channel vocoder in which, at the transmitting side, envelope values for a plurality of spectral channels which differ with respect to frequency position and width, as determined by a filter bank, and, if necessary, additional speech-associated parameters such as base speech frequency and voice characteristic, are derived in a vocoder analyzer and combined into a digital sum signal before being transmitted to a receiving side in successive pulse frames respectively embracing a single analysis interval. The digital sum signal is again divided at the receiver into the individual spectral channels and, if necessary, into the channels assigned to the additional speech-associated parameters, and the channel signals are subsequently supplied to a receiver vocoder synthesizer which, in turn, includes a filter bank corresponding to the vocoder analyzer at the transmitting side, and a pulse generator, and which emits a generated synthetic speech signal.
2. Description of the Piror Art
Channel vocoder transmission techniques are known, for example, from the publication IEEE Transactions on Audio and Electroacoustics, Volume Au-15, No. 4, December 1967, pp. 148-161. As a rule, such channel vocoders make use of the same filter bank in the analysis section and in the synthesizer section, given speech transmission in half-duplex operation; a favorable expense thereby occurs because the filter bank can be respectively switched and, therefore, must be present only once per device.
The transition from analog circuits to digital circuits being continuously undertaken in the course of advancing integrated technology also leads to the exclusive realization of channel vocoders in digital technology. As demonstrated, for example, by the publication IEEE Transactions on Acoustics, Speech and Signal Processing, Volume ASSP-29, No. 1, February 1981, pp. 13-23, particularly at Page 16, right-hand column, paragraph 1, infinite impulse response (IIR) filters and finite impulse response (FIR) filters are mentioned for the realization of a filter bank using digital techniques. However, the expense for such filters is considerable. This is particularly true for a non-recursive filter having finite impulse response because of the high ordinal number requirement. The digital recursive filters having infinite impulse response can indeed be realized for the state of pulse with a considerably lower ordinal number, but do not have the favorable properties of the FIR filters.
So-called "Switched capacitor filters" which are available in integrated construction and make due with a low installation volume can also be employed as digital filters. However, like all sampling filters, such filters have the disadvantage that their transfer function periodically repeats in the frequency range. In other words, the input signal for such a filter may not have any spectral components above half the sampling frequency. As practice has shown, the upper limit frequency of the input signal must have an even greater spacing from half the sampling frequency if disruptions are to be avoided. No problems thereby occur regarding the analysis section, since this spacing can be observed with certainty by simple techniques. In the synthesizer section, on the other hand, this requirement cannot be met without considerable additional expense as a result of the pulse-shaped excitation function. In other words, considerable analog filter structure must be employed in order to sufficiently suppress disruptive effects in the form of a chirping background noise.